The present invention relates to a bridgeless power factor improvement converter that does not have a diode bridge circuit that rectifies an input alternating current (AC) voltage.
As this kind of the bridgeless power factor improvement converter (also simply referred to as “a converter” below), Applicant has already proposed a converter that is disclosed in Japanese Patent Publication Number 2012-70490. The converter explained above is configured with an inductor, first and second switching elements, and first and second diodes. Specifically, a first terminal of the inductor is connected to one terminal of an AC power source. A first terminal of the first switching element is connected to a second terminal of the inductor. In the first diode, an anode is connected to a second terminal of the first switching element, and at the same time, a cathode is connected to the other terminal of the AC power source. A first terminal of the second switching element is connected to the second terminal of the inductor. In the second diode, a cathode is connected to a second terminal of the second switching element, and at the same time, an anode is connected to the other terminal of the AC power source.
In the converter that has the configuration explained above, the first switching element is a target of a high frequency switching at a positive half cycle of an input voltage (an AC voltage) that is supplied from the AC power source. The second switching element is a target of the high frequency switching at a negative half cycle. Further, during the positive half cycle of the input voltage in which the first switching element is the target of the high frequency switching, the second switching element stays in an OFF state. As a result, the second diode that cooperates with the second switching element also stays in the OFF state. On the other hand, during the negative half cycle of the input voltage in which the second switching element is the target of the high frequency switching, the first switching element stays in the OFF state. As a result, the first diode that cooperates with the first switching element also stays in the OFF state.
Therefore, during the positive half cycle of the input voltage in which the first switching element is the target of the high frequency switching, both ends of the second diode, which stays in the OFF state, are in a state in which an output voltage is applied to the cathode terminal with respect to the anode terminal as a reference. That is, a parasitic capacitor (also referred to as “a parasitic capacitance” below) of the second diode that stays in the OFF state is charged to the output voltage and at the same time, the parasitic capacitance of the first diode that stays in an ON state is discharged so as to be substantially zero volts. On the other hand, during the negative half cycle of the input voltage in which the second switching element is a target of the high frequency switching, both ends of the first diode, which stays in the OFF state, are in a state in which the output voltage is applied to the cathode terminal with respect to the anode terminal as a reference. That is, a parasitic capacitance of the first diode that stays in the OFF state is charged to the output voltage and at the same time, the parasitic capacitance of the second diode that stays in the ON state is discharged so as to be substantially zero volts.
As explained above, immediately after a point (a zero-cross point) at which the input voltage is switched from negative to positive or from positive to negative, because the diode that stays in the OFF state is shifted to the ON state, the voltage of both ends of the parasitic capacitance of the diode is discharged from the output voltage to substantially zero volts. Similarly, because the diode that stays in the ON state is shifted to the OFF state, the voltage of both ends of the parasitic capacitance of the diode is charged from zero volts to the output voltage. As a result, the voltage of a node connected between the first diode and the second diode fluctuates by a voltage value of the output voltage.
Therefore, when the switching element that is the target of the high frequency switching starts a switching operation at an normal ON time ratio (because the input voltage is low immediately after the zero-cross point, a large ON time ratio that is prescribed in advance is used) immediately after the zero-cross point, the voltage of both ends of the parasitic capacitance of the diode that stays in the OFF state is rapidly discharged from the output voltage to substantially zero volts, and at the same time, the voltage of both ends of the parasitic capacitance of the diode that stays in the ON state is rapidly charged to the output voltage. As a result, the voltage of the node connected between the first diode and the second diode rapidly fluctuates by the voltage value of the output voltage. Therefore, in this converter, a surge current is generated because the voltage of this node rapidly fluctuates by the voltage value of the output voltage. Further, EMI noise increases.
Accordingly, in the converter that is disclosed in Japanese Patent Publication Number 2012-70490, soft start control is used for a switching element that is a target of a high frequency switching immediately after a zero-cross point. The soft start control makes an ON time ratio increase gradually from 0% to a normal ON time ratio. As a result, the discharging and charging operations for the parasitic capacitance of the diode explained above are gradually performed. Thus, because the rapid fluctuation by the voltage value of the output voltage that is generated at the node of each diode is avoided, both the cause of the generation of the surge current and the biggest cause of the EMI noise are removed.
The conventional converter describe above, however, can be improved. That is, in the conventional converter, because the soft start control in which the ON time ratio is gradually increased for the switching element (a switch) needs to be performed immediately after the zero-cross point of the input voltage, the switching control for the switching element is complicated.